microRNAs are powerful regulatory molecules that are abundantly expressed in the developing and adult mammalian brain. Many primate-specific microRNAs are now known, making this class of genes attractive candidates for involvement in brain disorders such as schizophrenia and bipolar affective disorder. Little is know about the pattern of expression of microRNAs in the brains of normally developing humans or individuals with these disorders. Multiple possible mechanisms exist through which microRNAs could play a role in disease. This project will utilize a variety of experimental approaches to increase our knowledge of the normal expression and function of microRNAs in the developing and adult brain, and investigate the possible role of microRNA in the susceptibility to schizophrenia and bipolar disorder. First, we will quantify microRNA expression in normal human brain tissue from several developmental stages as well as from a matched set of samples from individuals with schizophrenia, bipolar disorder, and psychiatrically normal controls (35 individuals from each group) to provide baseline knowledge about microRNA expression in the normally developing human brain and search for evidence that some microRNAs are improperly expressed in schizophrenia and/or bipolar disorder. Second, we will search for population variability in the sequences of microRNAs and their targets in mRNAs of interest in schizophrenia and bipolar disorder, as these may be functional variants that increase disease risk. Third, we will test these candidate variants, as well as tagSNPs from a limited number of microRNA clusters, for association to schizophrenia and bipolar disorder using trios from the NIMH Genetic Initiative collection. Fourth, we will develop a list of 12 microRNAs of greatest interest based on evidence of involvement in the biology of schizophrenia and bipolar disorder from the prior three steps, and use a cell culture/transfection system to manipulate microRNA expression and validate mRNA targets. Fifth, we will characterize the temporal and spatial expression of the 12 microRNAs of interest and select targets. Sixth, we will use a cell culture/transfection system to systematically characterize the cell biological consequences of alteration in microRNA expression on neuronal development and functioning. A better understanding of microRNA function in the normal and pathological state could provide novel insights into new therapeutic approaches for schizophrenia and bipolar disorder.